1. Field of the Invention
The present invention relates to a device and a method for controlling synchronization where machine shafts are electrically-driven and in phased synchronism by plural, mutually accurate, electric motors. Such shafts may be found in conveyor systems, processing systems for resins and metals, and rotary presses.
2. Background of the Invention
When synchronization control is effected by electrically keeping mutual phases of plural electric motors (or mechanical shafts driven by those electric motors) unchanged, it is necessary to first match the "origins" (or starting points or reference points) of those electric motors or mechanical shafts, and then effect synchronization control.
For matching origins, a conventional method uses an origin detector provided on each electric motor or each rotating machine shaft to detect a machine origin. The electric motor is then interrupted, and the origins of all other electric motors are then detected. When detection of all origins for all motors is complete, synchronous operation is begun. In this way, a time of 30 to 50 seconds is required until the matching has been completed. This lengthens waiting time, causing poor working efficiency.
To solve such a difficulty, a recently proposed method matches the origins of plural electric motors in an improved manner, i.e., the matching of origins is achieved without once interrupting the electric motors during low rotational frequency operation.
FIG. 6 illustrates a prior art example in which matching of origins is effected during low rotational frequency operation of electric motors. In FIG. 6, two electric motors are exemplarily used in the matching of origins of plural electric motors for brevity of the description.
In FIG. 6, Mm, Ms1 are electric motors in a master section and a slave section, Pm, Ps1 are incremental encoders each coupled with the machine shafts driven by the electric motors, and Rm, Rs1 are the rotating machine shafts driven by the electric motors. Machine origins Gm, Gs1 are mounted on the machine shafts Rm, Rs1, which origins are detected by detectors Km, Ks1. The aforementioned master and slave electric motors Mm, Ms1 are driven respectively by drivers Dm, Ds1 and controllers Am, As1.
The aforesaid controller Am drives the electric motor Mm through the driver Dm following a rotational frequency instruction provided from a concentrated controller C by obtaining a rotational frequency signal through a rotational frequency detector Fm from a continuous pulse signal outputted by the aforementioned incremental encoder Pm, and feeding the rotational frequency signal back.
In the following discussion, the arrangement of the aforementioned controller As1 of the slave section in FIG. 6 will be described.
In the controller As1 a rotational frequency instruction is detected by the rotational frequency detector Ss1 from the pulse signal obtained from the aforementioned incremental encoder Pm of the master section. Further, a feedback rotational frequency of the slave section is detected from the incremental encoder Ps1 and the rotational frequency detector Fs1 of the slave section.
Herein, a cumulative counter Cs1 is cleared when the aforesaid detector Km of the master section detects the machine origin, and counts a pulse train of the aforesaid incremental encoder Ps1 of the slave section.
The counted value of the cumulative counter Cs1 is stored in a Z correlation distance memory area Zs1 with the aid of a switch RYs1, actuated when the detector Ks1 of the slave section detects the machine origin. More specifically, the stored value in the Z correlation distance memory part Zs1 indicates a Z correlation distance .DELTA..theta. obtained by measuring the phase difference of the mechanical origins of the master and the slave with the number of pulses of the aforesaid Ps1 of the slave section.
When the origins are matched, two electric motors are actuated and run at a low rotational frequency with a rotational frequency instruction of the aforesaid centralized controller C. In the slave section, the z correlation distance .DELTA..theta. is read out from the z correlation Zs1 in the operation at the low rotational frequency, and .DELTA..theta./.DELTA.T is calculated in order to adjust the time .DELTA.T and a correction value of the .DELTA..theta./.DELTA.T is subtracted from the rotational frequency instruction by the aforesaid rotational frequency detector Ss1 of the slave section. The correction is executed for the time .DELTA.T with the switch RYs2.
Two electric motors are matched in origins thereof by adjusting the rotational frequency of the slave section as described above, and are changed over to synchronization control and then accelerated into ordinary rotational frequency operation.
The prior art method and apparatus however suffer from difficulties that even when the origin matching is effected while operating the electric motors, the Z correlation distance .DELTA..theta. is detected by allowing the electric motors of the master and slave to rotate by one revolution or more, so that much time is required for the detection, and it takes 20 to 40 seconds until the origin matching is completed.
Further, in order to detect the Z correlation distance .DELTA..theta. it is needed that the rotational frequency of the master and slave electric motors are stabilized and they are operated at the same rotational frequency to the utmost, so that the origin matching must be done at a low rotational frequency, which causes a complicated adjustment.
Furthermore, when there are electric motors under operation and electric motors under interruption and the electric motors under interruption are started for synchronization control, the electric motors already in operation must be operated once at a low rotational frequency for the origin matching, and hence an operation procedure is complicated and much time is required.